Ink jet printing is a non-impact method for producing printed images by the deposition of ink drops in a pixel-by-pixel manner to an image-recording receiver element in response to digital signals.
There are various methods that are used to control the deposition of ink drops on the receiver element to yield the desired ink jet printed image. Ink jet printing systems are generally of two types: drop-on-demand (DOD) printing systems and continuous ink jet (CIJ) printing systems. For DOD printing systems, individual drops are projected as needed onto the receiver element. Common methods of controlling the ejection of ink drops in DOD printing include thermal bubble formations (thermal inkjet or TIJ) and piezoelectric transducers. The direction of the stream of droplets is controlled electronically in causing the droplets to print the desired image or information on the substrate surface without requiring contact between the printing device and the surface to which the ink is applied. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling. Objects, comprising substrates to which ink jet printing is well suited include, but are not limited to, containers for consumer products, currency, draft checks, envelopes, letterhead, identification cards, lottery tickets, bank cards, identification strips, labels, and other well known materials.
Drop-on-demand printing systems are widely used in home or consumer ink jet printers and slower commercial printers, both of which have been available for several decades. As the name implies, this type of ink jet printing uses a print head that ejects drops of ink only when signaled to do so by a digital controller.
CIJ printing systems generally comprise two main components, a fluid system (including an ink reservoir) and one or more print heads. Ink is pumped through a supply line from the ink reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in one or more linear arrays, under sufficient pressure to cause ink streams to issue from the orifices of the one or more print heads. Stimulations are applied to the one or more print heads to cause those ink streams to form streams of uniformly sized and spaced drops, which are deflected in a suitable manner, into printing or non-printing paths. Non-printing drops are returned to the ink reservoir using a drop catcher and a return line. Thus, in contrast to DOD printing systems, CIJ printing systems involve the use of a continuous stream of ink drops that are separated to discriminate between printing drops and non-printing drops. This discrimination can be accomplished by electrostatically charging the drops and passing the charged drops through an electric field. Charged drops are deflected by a charge field and are caught and returned to the reservoir of ink. Uncharged drops are printed on a substrate or receiver material. Some useful CIJ printing apparatus and print head fabrication are described for example in U.S. Pat. No. 6,588,888 (Jeanmaire et al.) and U.S. Pat. No. 6,943,037 (Anagnostopoulos et al.).
More recent innovations in CIJ printing systems include the use of silicon print head nozzles with heaters built into them. These print head nozzles respond to an electronic signal and change the physical characteristics of drops being ejected. Heated drops “catch up” in velocity with non-heated drops in the emission space. Multiple drops can combine to form a “printed drop” that is printed onto the substrate or receiver material. Single drops are caught by an air stream and returned to the ink reservoir for future use.
Commercially available CIJ inks are mostly aqueous dye-based inks that exhibit a number of problems. In such dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on coated glossy paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness and poor smear resistance.
Colored pigment-based inks have been recommended as a way of addressing limitations of dye based inks. In pigment-based inks, the colorant exists as discrete particles. Pigment dispersions known in the art include self-dispersing pigment dispersions, dispersant stabilized pigment dispersions and encapsulated pigment dispersions. For non-self-dispersive pigments, the pigment particles are typically treated with addenda known as dispersants or stabilizers that serve to keep the pigment particles from agglomerating or settling out. Useful pigment-based inks for CU printing systems are described for example in U.S. Patent Application Publications 2010/0304028 (Sowinski et al.) and 2011/0122180 (Cook et al.).
In general, such pigment-based inks can comprise a wide variety of colored pigments that are chosen depending upon the specific application and performance requirements for the printing system and desired printing results. For example, such pigments can include, but are not limited to, carbon black or other black pigments, red pigments, green pigments, blue pigments, orange pigments, violet pigments, magenta pigments, yellow pigments, and cyan pigments. The printed images using such pigment-based inks are generally desired to have a visual density of at least 0.5.
Colorless or invisible aqueous ink jet printer ink compositions containing various fluorescing pigments, and optionally colored non-fluorescing pigments, are described in U.S. Pat. No. 8,349,211 (Cai et al.) and in copending and commonly assigned U.S. Ser. No. 13/769,504 (filed Feb. 18, 2013 by Cook).
Ink jet printer ink compositions containing colorants or invisible components are classified as either pigment-based, in which the colorant or invisible components exist as pigment particles suspended in the composition, or as dye-based, in which the colorant or invisible component exists as a fully solvated dye species comprising one or more dye molecules.
A pigment is generally more desirable because it is more resistant to fading than a dye. However, pigments have a number of drawbacks. Pigments should be physically milled to produce particles of a small enough size to provide sufficient colloidal stability to the particles. If the pigments particles are too large, resulting light scattering can have a detrimental effect on optical density and gloss in the resulting printed image.
A second drawback of pigmented ink compositions is their durability after printing on the receiver elements, especially under conditions where abrasive forces have been applied to the printed images. Furthermore, the printed images on the receiver elements are susceptible to defects at short time intervals, from immediately after printing to several minutes later while the ink jet printer ink compositions are drying. In addition, the durability of the dried printed images is also subject to environmental factors such as temperature and humidity that, under certain circumstances, can degrade image durability.
To solve these various problems, pigmented aqueous ink jet ink compositions have been formulated with various polymers, dispersants, and other addenda to provide durable images that can withstand post printing physical abuse and environmental conditions. For example, such compositions have been formulated with various acrylic polymers, polyurethane, or mixtures thereof as described for example in U.S. Pat. No. 8,187,371 (Brust et al.). The described polyurethanes have an acid number of from 60 to about 130. Similar compositions are described in U.S. Patent Application Publication 2008/0207811 (Brust et al.) and U.S. Pat. No. 8,192,008 (Brust et al.). Polyurethane/urea polymers are described as additives in U.S. Patent Application Publications 2009/0169748 (House et al.), 2009/0169749 (Brust et al.), and 2012/0050380 (Falkner et al).
Although the noted aqueous ink jet ink compositions provide improvements in ink jet printed images, when the polyurethane has a high acid number, it can absorb too much moisture in high humidity environments. Yet, if the acid number is too low, ink jetting becomes poor such that it is non-uniform and unacceptable at high jetting speeds.